This invention relates to a burner for heat production and more particularly to a burner adapted for placement in an air duct for heating an airstream and for reducing pollutants flowing in the airstream.
Gas-fired duct burners are frequently used to raise the temperature of an air or turbine exhaust gas (TEG) stream or to incinerate gaseous or particulate matter in an airstream. However, a duct burner operating in a TEG stream may act as a carbon monoxide or oxide of nitrogen generator rather than an incinerator. The quantity of carbon monoxide incinerated by the duct burner usually is inversely related to the quantity of oxides of nitrogen generated. For example, as the amount of oxygen in the TEG stream decreases, carbon monoxide levels increase and oxide of nitrogen levels decrease. Additionally, at low firing rates, carbon monoxide levels are high whereas oxide of nitrogen levels are low. As the firing rate increases, carbon monoxide levels decrease and oxide of nitrogen levels increase, but as the firing rate is increased further, carbon monoxide levels increase and oxide of nitrogen levels decrease. As a result, it is difficult to design a burner which emits minimum levels of both pollutants.
To further complicate the problem, the exhaust gas from present-day gas turbines has a higher temperature (typically 1000.degree. F.) and a lower oxygen content (typically 14%) than older turbines. Furthermore, many gas turbine installations are using steam injection as a means for controlling the oxides of nitrogen generated by the turbine combustor and to enhance cycle efficiency. However, the injection of steam further reduces the oxygen content of the TEG stream, and this often results in unacceptably high levels of carbon monoxide. Accordingly, it has become much more difficult for known duct burner designs to meet government standards.